I'm another fan of the 3rd gen 4runner. I'm just shy of 200k on mine. I've had it since about 100k miles.
On the highway at 65 you should get 20mpg pretty easily. I can get around 22mpg at 70mph. Around town it's only around 17ish. One thing I noticed that hurt my mpg was putting larger heavier tires on. If you stick with street tread P-metric tires stock size, you'll probably get the best mileage.
There are a few problem areas of note:
Rear springs sag over time. I put firestone air bags in mine (they go inside the factory coil springs). I've been really happy with them. And they are cheap: $100 for the kit.
Front brakes. The 4runners are notorious for front brake shimmy. I believe it has to do with the rear brakes going out of adjustment causing overworking of the front brakes. I have done the tundra brake conversion and I would recommend not doing it. I don't think it solves the problem and is more or less a waste of money. Fix your rear brakes and put new rotors and pads up front and you'll be happy.
o2 sensors. The o2 sensors can go bad without throwing a code. Your mpg will drop by about 3-4mpg over time. New o2 sensors are $60ish and will fix the low mpg issue. You can google it for more info.
Of course there's the timing belt that needs to be done every 90k. My last one I did a timing belt, water pump, and replaced all the idler pulley bearings, as well as did a full transmission flush and replaced all the rubber coolant hoses. I think that the 2nd timing belt is a great time to do all of that and know that it's good for another 200k.
The leather seats are cracking and worn out.
Radiators are prone to leak between the transmission fluid and coolant compartments. Google 4runner and pink milkshake. Mine is still working as normal. I've just read that that's been an issue.
There are times I wish I had more power. Mostly on long mountain passes towing my sailboat.
Other than that, my 4runner has been exceptionally reliable. Believe it or not, at 200k miles, it still has the factory u-joints. The only non-routine item I've replaced is the muffler a week ago and it's due for shocks (which will be the first time they've been replaced). And the power antenna doesn't work anymore. Every window still works like new, every lock, every switch, you name it, the thing has been exceptionally reliable.